With the recent development of portable devices such as mobile phones, notebook computers, and camcorders, demand for secondary batteries such as nickel-metal hydride (Ni-MH) secondary batteries and lithium secondary batteries is increasing. In particular, a lithium secondary battery using lithium and a non-aqueous solvent electrolyte has a high possibility of producing a battery having a small size, light weight, and high energy density and is being actively developed. In general, in preparation of a lithium secondary battery, transition metal oxides such as LiCoO2, LiNiO2, and LiMn2O4 are used as positive electrode materials, and lithium metal, carbon, and the like are used as negative electrode materials, and an organic solvent containing lithium ions is used as an electrolyte between the two electrodes.
However, lithium secondary batteries using lithium metal as a negative electrode are liable to generate dendritic crystals with repeated charging and discharging, and as a result, there is a great risk of short-circuiting. Thus, lithium secondary batteries, which use carbonized or graphitized carbon as a negative electrode and a non-aqueous solvent containing lithium ions as an electrolyte, have been put to practical use. However, since carbon-based negative electrode materials have a large irreversible capacity, initial charge/discharge efficiency is low and capacity is reduced. In addition, lithium may be precipitated on the surface of carbon during overcharging, causing problems in safety.
On the other hand, a lithium titanium oxide has recently attracted attention as a negative electrode material used in lithium ion batteries. The lithium titanium oxide has an operating voltage of 1.3 to 1.6 V and a reversible capacity of about 170 mAh/g, which is a higher operating voltage and a lower reversible capacity compared to conventional carbon-based negative electrode materials. These features may be disadvantageous when the lithium titanium oxide is used as a negative electrode material. However, when using the lithium titanium oxide, high-speed charging and discharging is possible, almost no irreversible reaction occurs (initial efficiency of 95% or more), and the heat of reaction is very small, which provides excellent safety. These features may be advantageous when the lithium titanium oxide is used as a negative electrode material. In addition, the theoretical density of carbon materials is as low as about 2 g/cm3, whereas the theoretical density of Li4Ti5O12, which is a lithium titanium oxide, is as high as about 3.5 g/cm3. Thus, Li4Ti5O12 has a similar capacity per volume compared to the carbon materials.
Conventionally, lithium titanium oxides are used as high input/output materials in the way that the size of lithium titanium oxide particles is miniaturized to enlarge the active surfaces of the lithium titanium oxide particles, thereby accelerating the diffusion rate of lithium.